1. Field of Invention
The present invention relates to an image processing method, an image processing apparatus, and a projector.
2. Description of Related Art
In the related art, when a moving image is displayed in an image-display apparatus, the moving image is subjected to tone-conversion processing, luminance-correction processing, or the like, which is adaptive to the situation of the moving image. In an exemplary related art method of performing such processing, an APL (average picture level) of a single frame, that is, an average luminance level of an image is used. With this method, the tone-conversion characteristic of the moving image is changed in accordance with the APL; for example, a large number of tone levels are allotted at the low-tone level side of the moving image when it is estimated that the image has a low APL and is hence dark as a whole.
However, when the image is subjected to adaptive processing only with the APL as a characterizing amount, the tone-conversion characteristic is decided in accordance with only the contrast of the screen, as a result, image processing suitable for the scene is not correctly performed. Also, with this APL-adaptive tone-conversion processing, even when an observer does not notice a change in the scene (hereinafter, referred to as a scene-change) with his or her eyes, the tone-conversion characteristic varies as the APL varies. Accordingly, the luminance of the image varies due to the variation in the tone-conversion characteristic or the like, thereby resulting in flickering and thus causing a risk of forming an unnatural image.
Therefore, in order to adaptively perform a tone conversion, a luminance correction, and the like for each scene of the moving image, it is desirable to detect a natural scene-change which agrees with that noticed by an observer with his or her eyes. An exemplary method of detecting a scene-change by using a luminance histogram is disclosed in Japanese Unexamined Patent Application Publication No. 8-331504. Hereinafter, a luminance histogram means a frequency distribution diagram of luminances of respective pixels in a single frame, where the horizontal axis of the diagram is the luminance level and the vertical axis is the number of pixels (frequency) having the corresponding luminance level.
In the method using such a luminance histogram, first, the histogram of each frame is computed. Next, a difference in frequencies at each luminance level for each luminance histogram between comparing frames is computed. Then, the sum total of the differences (in absolute value) of each frame is computed. Finally, when the sum total of the differences exceeds a predetermined threshold, it is determined that the scene-change between the comparing frames has occurred.